The recent international concern regarding the climate change has prompted carbon cap and trade agreements and sparked interest in emission free energy productions. In addition to its many uses in the chemical and petrochemical industries, hydrogen has the potential of being a sustainable “emissions free” energy carrier. Steam reforming and coal gasification, which are already common industrial processes, can be coupled with CO2 capture and sequestration as a means of producing hydrogen on an industrial scale with limited carbon emissions.
Palladium membrane reactors are especially suited for catalytic membrane reactors as the reaction and separation steps are combined while producing high purity hydrogen gas on the permeate side of the membrane and pressurized CO2 on the retentate side, lessening the amount of energy required for carbon capture. Separating hydrogen gas from CO2 and other reforming by-products is currently done by pressure swing adsorption (PSA), but membrane reactors equipped with hydrogen selective Pd membranes have the potential to produce high purity hydrogen gas at lower temperatures by extracting the H2 simultaneously with H2 production, thus greatly reducing the energy expended in reaching a high conversion.
Composite Pd and Pd alloy membranes deposited on porous metal supports reduce the amount of Pd needed for separation, thus increasing the H2 permeance and profitability, and have both the strength and structural integrity needed to withstand the large pressure differences needed on the industrial scale. Depositing the Pd with the electroless deposition method yields a hard film which uniformly covers complex shapes, and is relatively easy to scale up. The good adhesion between the metal substrate and the hydrogen selective Pd or Pd alloy layer ensures that the membrane will not detach during operation and thermal cycling due to differences in coefficients of thermal expansion or lattice expansion in a hydrogen atmosphere, as is prone to happen with ceramic supports, or porous metal supports which have been heavily modified with a layer of ceramic material.
However, small quantities of H2S present in the gas stream poison Pd membranes causing either a drastic reduction of the hydrogen permeance or irreparable damage to the membrane by forming a Pd4S layer. Pd/Cu alloys have generated much research, not only because they do not exhibit hydrogen embrittlement even at room temperature but also because Pd/Cu alloys are more resistant to H2S poisoning than pure Pd when the Cu composition is in the fcc region of the Pd/Cu phase diagram. FIG. 1 is a Pd—Cu phase diagram overlaid with the relative permeability of Pd/Cu membranes to Pd membranes as a function of Cu content at 350° C. One of the disadvantages of fcc Pd/Cu membranes is their low permeance in comparison to Pd and other Pd alloys due to the decreased solubility of H2 in Pd/Cu alloys. The addition of only 10 and 30 wt % Cu decreased the permeance of Pd membranes, respectively by 35% and 85% at 350° C. See U.S. Pat. No. 3,247,648. An even larger decrease was seen at 540° C. with the permeance decreasing by 20 and 40% at concentrations of merely 5 and 10 wt % Cu respectively. Grashoff, G. J. et al., The purification of hydrogen: A review of the technology emphasizing the current status of palladium membrane diffusion, Platinum Met. Rev. 27 (1983) 157, the relevant teachings of which are incorporated herein by reference in their entirety.
The significant hydrogen permeance decrease seen in fcc Pd/Cu alloy membranes forces ultra-thin Pd/Cu membranes to be fabricated in order to achieve permeances which are comparable to pure Pd membranes or the more highly permeable Pd/Ag membranes. The relatively wide pore size distribution of porous metal supports makes reducing the Pd/Cu membrane thickness more difficult than with ceramic supports whose pore size distribution is more easily controlled. In addition, the selectivity and structural integrity of such ultra-thin Pd/Cu membranes is then more likely to deteriorate quickly.
In order to lessen the impact on the permeance by alloying Pd with Cu and while still retaining the sulfur resistance, Pd/Cu membranes with an fcc alloy only on the top layer have been fabricated and characterized by plating and annealing the bi-layers of Pd and Cu, thereby retaining the H2S resistance without as much of a decrease in the permeance as with a homogeneous Pd/Cu membrane of the same surface concentration.
In addition, recent high temperature x-ray diffraction (HT-XRD) studies of the annealing of Pd/Cu bi-layers disclosed that very long annealing times in H2 were necessary to produce the fcc alloy on the surface and for the less resistant bcc phase to disappear. Less lengthy annealing times were achieved at temperatures as high as 600° C. but the exposure to such high temperatures would be damaging to membrane selectivity, even for short time periods.
Thus, there is a need in the art for an improved method of forming a membrane that alleviates the disadvantages described above. In particular, there is a need for a method of forming a membrane that is resistant to H2S poisoning while retaining a high H2 permeance. In addition, there is a need for a method of forming such a membrane that can be annealed at lower temperatures and for shorter periods of time.